Desulfurization of flue gas

ABSTRACT

REMOVAL OF SULFUR DIOXIDE FROM HOT FLUE GAS BY PASSING THE FLUE GAS IN CONTACT WITH POTASSIUM FORMATE, SODIUM FORMATE, OR AMMONIUM FORMATE, IN EITHER A MOLTEN STATE OR IN AQUEOUS SOLUTION, AT A TEMPERATURE ABOVE 140*F., WHEREBY THE SULFUR DIOXIDE AND THE FORMATE REACT TO FORM PRINCIPALLY THIOSULFATE.

United States Patent ABSTRACT OF .THE DISCLOSURE Removal of sulfurdioxide from hot flue gas by passing the flue gas in contact withpotassium formate, sodium formate, or ammonium formate, in either amolten state or in aqueoussolution, at a temperature above 140 FL,whe'reby'the sulfur dioxideand' theformate react to form principallythiosulfate.

CROSS-REFERENCES TO RELATED APPLICATIONS This application is acontinuation of application, Ser. No. 879,223, filed Nov. 24, 1969, andnow abandoned which is a continuation-in-part of application, Ser. No.667,479, filed Sept. 13, 1967 (now abandoned). Other relatedapplications, filed on Nov. 24, 1969, describing and claiming certainsubject matter hereinafter disclosed are an application, ,Ser. .No.'879,244, now Patent No. 3,584,042, filed by P. M. Yavorsky-and E. Gorinentitled Conversion ,ofThiosulfate to Formate and an application, Ser.No. 879,225 filed by N. J. Mazzocco, E. Gorin and P. M. Yavorskyentitled Regeneration of Formate From. Thiosulfate (now abandoned, butreplaced by a continuation-in-part application, Ser. No. 47,040, filedJune 17, 1970, now Patent No. 3,592,850).

BACKGROUND OF THE INVENTION (1) Field of the invention The presentinvention relates to a method of removing sulfur dioxide. from gases,particularly from'industrial gases which may contain only a very smallamount of sulfur dioxide, such as flue gases which are formed in theburning of sulfur-containing coal or oil, and also in certainmetallurgical processes.

(2) Description of the prior art Wet scrubbing of flue gas withsulfur-absorbing reagents is an extremely effective method of removingS0 For example, the use of soda ash solutions has been shown to etfectremoval of 95 percent in full-scale plant tests at the Battersea PowerStation, England, from flue gases containing ca. 0.1 volume percent S0Similarly, high eificiency has been achieved in pilot tests on removalof S0 by washing with alkali sulfite solutions. See Johnstone, H. F.;Read, H. J.; and Blankmeyer, H. C., Ind. Eng. Chem. 30, 101 (1938). Wetscrubbing has also been investigated with disposable" Weakly alkalinereagents, i.e., water-lime slurries. Tests at the Battersea PowerStation indicated highly efiicient sulfur removal in this way, butalmost as good elimination was obtained with the wateralone, apparentlydue to the natural alkalinity it contained.

"ice

The alkali metal and alkaline earth salts of formic acid in aqueoussolutions have been proposed as wet absorbents of sulfur dioxide(Japanese Patent No. 172,814, issued May 31, 1946). The sulfur dioxideis absorbed at ambient temperatures below 50 0., according to thefollowing illustrative reaction:

The patent further teaches that the sulfur dioxide may be released bysimply heating the solution to a temperature above 55 C., and preferablynearer C.

The wet scrubbing methods which operate at low ambient temperaturessuffer from the so-called plume problem. The low buoyancy of the gasresulting from scrubbing at substantially ambient temperatures requiresthat almost quantitative removal of S0 be effected. Otherwise, due topoor dispersion of the flue gas and the resultant plume, the groundlevel contamination may be even worse than without scrubbing. Thescrubbed flue gas may be reheated to elevated temperatures beforerelease to the atmosphere, but such reheating is expensive.

In most utility stations, the flue gas exiting from the air preheaterand entering the stack has a temperature in the range of 250350 F. Thehigh temperature facilitates plume rise. It would be desirable to scrubthis gas for S0 removal without substantial cooling, to insure plumerise without costly reheat. Accordingly, the scrub bing medium shouldpreferably be a liquid which is effectively operative in, or as nearthis temperature range as possible.

SUMMARY OF THE INVENTION In accordance with the present invention, aprocess is provided for removing sulfur dioxide from hot flue gas whichcomprises passing the hot flue gas in contact with sodium formate,potassium formate, or ammonium formate, preferably in a liquid state, ata temperature above F., but below the decomposition temperature of theformate, and preferably below 475 F. The formate may be dissolved in anychemically inert solvent which is liquid within the above temperaturerange. Water is the preferred solvent when a solvent is used. However,the salt may be used in the molten state rather than in a solvent.Likewise, suitable mixtures of the formates may be used. For example, wehave found a suitable wet absorbent to be the eutectic mixturecontaining 96 percent potassium formate and 4 percent sodium formatewhich melts at 320 F., 13 F. below the melting point of the purepotassium salt. The sulfur dioxide in the flue gas reacts very rapidlyand completely with the formate to form principally the correspondingalkali metal thiosulfate at temperatures above 140 F. and up to 475 F.,as shown in the following equation:

where M is Na, K or NH.,.

The reaction is an oxidation-reduction reaction rather than an acid-basereaction, as set forth in the above-cited Japanese patent. The reactionof formate and sulfur dioxide under the recited conditions is thus amethod of making the thiosulfate.

At temperatures above 475 F., secondary reactions of the thiosulfateformed by reaction (1) occur, and increase in rate as the temperature isincreased. Among such secondary reactions is the following:

It is undesirable in most instances to replace the S in the flue gas byH 8. Accordingly, the temperature is generally maintained below 475 F.during the S0 absorption cycle.

Significant quantities of M 8, MHS, minor amounts of M 80 andoccasionally some M 80 and elemental sulfur may also be produced. Theamounts of these by-products depend on the conditions employed in theabsorption or scrubbing step. The formation of by-products is minimizedby reduction of temperature and residence time. At temperatures below400 F., the formation of H S in the scrubbing reaction appears to becompletely inhibited, especially when M CO is present in the formate.For this reason, its presence may be desirable, though it is clear thatcarbonate is not necessary for absorption of S0 by the formates.

Both sodium thiosulfate and potassium thiosulfate are useful per se aschemical agents in industry. In particular, they are useful as fixingagents in developing solutions used in photography, sodium thiosulfatebeing the wellknown hypo. Ammonium thiosulfate may be readily convertedto the sulfate which is used as a fertilizer. Thus, the thiosulfateproducts of Reaction (1) may be marketable as such, after suitablepurification.

If, on the other hand, regeneration of the thiosulfate product toformate is desired to permit its reuse in the SO -absorbing system, itmay be accomplished by reduction (at least in the case of sodium orpotassium thiosul fate), as will be more fully discussed later. However,such regeneration does not form part of the present invention which isconcerned solely with the remarkable effectiveness of the selectedformates as absorbents for S0 at elevated temperatures.

Generally speaking, we prefer to use aqueous formate solutions ratherthan the molten salts. The three basic advantages of the aqueous systemare (1) complete elimination of evolution of H 8 into the scrubbed linegas (which sometimes happens in the case of the melt system at hightemperature), (2) operation under conditions where no insoluble saltsare precipitated, and (3) a much less serious corrosion problem.

The aqueous formate solutions may be conveniently used within thetemperature range 140 to 250 F., preferably Within the range 150 to 225F. The higher the operating temperature, the higher the saltconcentration that is required to prevent evaporation. Because of thevery high solubility of potassium and ammonium formates, concentrationsup to 90 weight percent may be used, but 70 to 85 weight percent ispreferred over the temperature range 170 to 200 F. In the case ofaqueous sodium formate, lower concentrations are used because of thelower solubility of the sodium salt. Concentrations 4 of about to weightpercent are preferred over the temperature range of about 60 to 170 F.

The use of the formates in molten form does permit eflicient removal ofS0 from hot flue gas Without any cooling of the flue gas. The meltingpoint of potassium formate is 333 F. The S0 absorption may be conductedat a temperature above 350 F., but preferably below 475 F. Above 475 F.,some H 8 is evident, thus defeating the primary purpose of desulfurizingthe flue gas. Somewhat lower temperatures may be achieved in potassiumformate melt systems if other salts are added which depress the meltingpoint of the potassium formate. The addition of small amounts of sodiumformate effects such a melting point depression. Likewise, operation atsomewhat reduced temperatures is possible by addition of small amountsof water. For example, we have found that 98% KCOOH2% H O forms ahomogeneous liquid at 310 F. and has a sufi'iciently low vapor pressureof water at the above temperature to be useful as a scrubbing fluid forflue gas.

The melting point of sodium formate is 489 F., which is below itsinitial thermal decomposition point of 600 F. However, to avoidintroduction of substantial amounts of H 8 into the flue gas, it isnecessary to operate with sodium formate at temperatures below 475 F.Thus, its admixtures with other salts or inert solvents which depressthe melting point are used.

The melting point of ammonium formate is 240 R, which is above thetemperature at which it starts to decompose, i.e. 170 F. Accordingly,for all practical purposes, its use as absorbent is limited totemperatures below 170 F. and to solutions, as distinguished from melts.

The process of the present invention is directed preferably to the useof the recited formates in a liquid state as wet absorbents for S0 Theprocess is founded on our discovery that S0 reacts with the recitedformates at temperatures above 140 F. to form the correspondingthiosuifates. This new reaction is obviously not dependent upon thephysical form of the reactants. Hence, the recited formates, suitablysupported on inert carriers, could serve as dry absorbents for S0 asdistinguished from the wet absorbents of the process of this invention.

EXPERIMENTAL RESULTS TABLE I Feed gas (volume Conditions SO; absorbentsystem (gms) percen Gas rate Run dura- Temp. KGOOH K 002 E10 S0 N2(liters/hr.) tion (hrs) F.)

0. 83 1 94. 0 34. 0 3. 00 450 0. 83 1 94. 0 34. 0 3. 00 450 l. 11 98. 8934. 0 1. 835 350 5. 56 94. 44 34. 0 0. 583 350 5. 56 94. 44 34. 0 0. 45035,0 0. 26 2 81. 23 34. O 2. 50 225 0. 26 Z 81. 23 34. 0 0. 667 225 3.00 97. 00 34. 0 l. 175 3. 00 97. O0 34. O 2. 00 150 5. 56 94. 44 34.0 1. 300 450 5. 56 94. 44 34. 0 l. 250 450 5. 56 94.44 34. 0 1. 200 3505. 56 94. 44 34. 0 0.700 350 5. 56 94. 44 34. 0 l. 150 350 5. 56 94. 4434. 0 1. 000 350 5. 56 94. 44 34. 0 1. 250 350 4. 10 95. 90 68. 0 5. 22250 1 Feed gas also had 0.97% 002, 0.93% 00, 3.3% Oz. 2 Feed gas alsohad 12.1% 00 6.41% 0 TABLE II Cumulative ofi-gas analysis Scrubbingresults Molar ratio Percent Total S S: C0: H28 C02 011/ S02 added to(gms.) (gms.) (gms.) S0 in absorbed melt (gm) 1 0. 00 100. 0 1. 20 10,00 100. 0 1. 20 a 0. 00 100. 0 0. 99 0. 00 100. 0 1. 57 0. 00 100. 0 1.21 1 0. 00 100. 0 0. 32 1 0. 00 100.0 0. 084 0. 00 100. 0 2. 55 0. 00100. 0 2. 91 0. 00 5. 26 0. 09 1. 1 100. 0 3. 50 0. 00 4. 65 0. 00 1. 0100. 0 3. 38 0. 00 4. 45 Trace 1. 0 100. 0 3. 21 0.00 2.60 1.0 100.01.89 0o 0. 00 4. 27 1 0.32 1. 0 100.0 3. 1l} 0.00 3. 71 1. 0 100. 0 2.70 0. 00 4. 65 0. 00 1. 0 100. 0 3. 3 5. 08 21. 60 0. 00 1. 0 3 75. 615. 7 2

1 Oxygen analyzed in oft-gas. No change. Oxygen out=oxygen in. 2Products of Runs 13 and 14 combined for analyses. l 3 SO: absorption wasincomplete due to poor contact of gas with llqllld.

TABLE IIL-SCRUBBING PRODUCT ANALYSES 1 Total sulfur calculated from theSO; absorbed.

' Cumulative sulfur-obtained by analysis.

3 Balance of sulfur in other sulfur compounds, e.g. K23 and K2804. 4Products of Runs 13 and 14 combined.

The S0; scrubbing runs presented in the above tables are in two groups.Runs 1 through 8 were made to test the efiicacy of potassium formate,either alone or in admixture with potassium carbonate, for S0absorption. This was done for molten salts (Runs 1 to 5 inclusive) andfor concentrated aqueous solutions (Runs 6"to 8). Then, when theeflicacy of all systems was found to be high, it was desirable to findout what the products of the scrubbing reactions were. Hence, for thesecond group, Runs 9 through 17, analyses were conducted on theproducts.

It is clear from the data for the first group of runs (Runs 1-8) inTables I and II, that blends of potassium carbonate in molten potassiumformate absorb S0 very efficiently (100%) at 450 F. (Run 1) and 350 F.(Runs 3 and 4). The aqueous solution of formate and carbonate also is aneflicient scrubber (Run 6) even at the lower temperature of 225 F.However, it is also immediately clear that the companion runs (Nos. 2,5, 7 and 8) that have only formate and no carbonate, are just aseflicient, with 100% absorption of the S0 from simulated flue gases andfrom SO /N blends. When high concentrations of 80;, were used in thefeed (Runs 4 and 5), the gas feed tube plugged where it entered themelt. This indicates that the reaction with S0 is very rapid, becausethe 10- calized solidification of the relatively insoluble productsoccurs at the gas inlet site. Also, the maximum residence time could nothave been more than a second or two, which was the observed bubblerise-time in the scrubbing medium. Still, complete removal of S0 wasalways-attained, with the exception of Run 17. The incomplete absorptionin Run 17 resulted from poor liquid-gas contact because the gas wasintroduced above the liquid, rather than bubbled through.

As shown in Runs 3, 4 and 5, the evolved CO was measured and found to beequal (molar-wise) to the S0 absorbed when carbonate was present, butjust slightly more than when only formate was present. Later similarruns at 350 F. (12 through 16) show the molar ratio of CO -Ofi OVerSO-in to .be 1.0. Thus, evolution of CO cannot be used to distinguishwhether the reaction is with carbonate or formate in cases where bothare present, since it evolves from formate as well as from the expectedcarbonate reaction,

In Runs 1 and 2, CO was analyzed for in the off-gas. No increase overfeed CO content was found. Thus, CO is not a reaction product. Likewise,analyses for oxalate in the salt product showed that it is not producedfrom formate. Oxygen was present in the feed gas for Runs 1, 2, 6 and 7.For these runs, 0;, analysis of the off-gas showed no reduction of 0 ascompared to the feed. Thus, residual O in flue gas will not adverselyreact with formate-based liquid scrubbers.

The analyses of sulfur forms in the scrubbing products, as presented inTable III, consistently show that thiosulfate isthe major product fromreaction of S0 with formate. Coupling this fact with the observationthat a mole of CO evolves per mole of S0 absorbed indicates that themajor absorption reaction with formate is reaction (1), namely:

At 350 F. (Run 14) and at 450 F. (Run 10), both with molten formateonly, some H S was found in the offgas from scrubbing. This, of course,is undesirable in scrubbed stack gas, even if it is only a few percentof the original S0 content. No H 8 evolution was observed when K CO waspresent, or when aqueous formate solutions were used at 250 F. or less(Run 17). In general, the sulfur distribution is altered in thescrubbing products when carbonate is present. The proportion ofthiosulfate is then much less, while M S increases. Also, note that thecarbonate is not consumed, as seen in Runs 11 and 16, since the amountof carbonate in the product is equal to that in the initial scrubbingblend. Thus, the reactivity of formate with S0,. greatly exceeds that ofcarbonate.

Similar results to those obtained with potassium formate were obtainedwith both sodium formate and ammonium formate, when used underconditions which recognized the diiferences in their respective physicalproperties, namely melting points, decomposition points, andsolubilities, aspreviously set forth. In all cases where goodgas-to-liquid contact was obtained, essentially per cent absorption of$0 was obtained, with thiosulfate being always the principal product.

DESCRIPTION OF THE DRAWINGS For a better understanding of our invention,its objects and advantages, reference should be had to the accompanyingdrawings in which FIG. 1 is a schematic fiowsheet of the preferredembodiment of our process; and

FIG. 2 is a schematic flowsheet of a modification of the process of thisinvention. t

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 ofthe drawings, there is shown a fiowsheet of a process which includes thepreferred embodiment of the present invention, namely the absorption ofS by means of formates in the liquid state, as well as the preferredembodiment of a regeneration procedure. We prefer to regenerate theformate from the product thiosulfate, rather than recover the productthiosulfate for sale or other use, as earlier described. Accordingly, wehave included the specific regeneration procedure we prefer to use, eventhough it does not form part of the present invention, and is more fullydescribed and claimed in application, Ser. No. 879,225 filed Nov. 24,1969 by Messrs. Nestor J. Mazzocco, Everett Gorin and Paul M. Yavorsky(now abandoned, but replaced by a continuation-in-part application,

trated aqueous potassium formate (e.g. 85%,) is fed into the top of thescrubber through a conduit 14.. The scrubber may be any conventionalgas-liquid scrubbing tower designed to assure contact of the SO-containing gas at elevated temperatures with the selected formatein aliquid state. We prefer to use a jiggling bed of marbles through whichthe gas and liquid pass in countercurrent flow relationship. Thetemperature within the scrubber is preferably maintained between 170 and200 F. when concentrated aqueous potassium formate is the absorbingagent. This temperature range has the advantage of eliminating the needfor reheat of the scrubbed gases when they are released to theatmosphere. The scrubbed gas, freed of S0 or substantially so, isdischarged through a stack 16 as clean stack gas.

The relative amounts of SO -containing gas and formate passing throughthe scrubber are regulated to provide for considerable excess of theformate, so that less than percent by weight of the formate is convertedto the thiosulfate in accordance with the reaction expressed by Equationl. Accordingly, the major constituents of the efiluent liquid streamleaving the bottom of the scrubber through conduit 18 are aqueouspotassium formate and potassium thiosulfate. These are pumped by conduit18 through a filter 19 (to remove any entrained solids, e.g. ash orinsoluble lay-products) to a stirred Reductor vessel 20 wherein theexcess formate is used to reduce the thiosulfate to K CO and KHSaccording to the following reaction:

( K S O +4KCOOH=2KHS 1 The temperature within the Reductor 'ismaintained at about 540 F. while the pressure, which is self-generated,is held at about 500 p.s.i.g. The required reaction time is about 20minutes. The gaseous product-Co ls discharged from the Reductor througha'pipe 22. .1

The products from the Reductor which areinaqueous The gaseous H 8 isdischarged through a stack'30 to a suitable recovery or processing plant(not shown). The

aqueous solution of K CO' is pumped through a conduit 32 to" a stirredFormate Regenerator vessel 34 where the aqueous K CO is reconverted toaqueous KCOOH by reaction with CO introduced through a conduit 36,according to the following equation:

The temperature maintained in the Regenerator is about 540 F., and thepressure held at about 1000 p.s.i.g. The residence time is about onehour. The gaseous product CO is discharged through a stack 38, while theregenerated aqueous formate is recycled to the scrubber through .theconduit 14, after suitable adjustment of its concentration to percent inthe aqueous solution.

DETAILED DESCRIPTION OF ALTERNATIVE EMBODIMENT Referring to FIG. 2, analternative embodiment of the process of the present invention is shownin which the formate in molten form is used as the Soy-absorbing agent.A diiferent regeneration procedure from that shown in FIG. 1 is used todemonstrate that there are other ways available for regenerating theformate. Numeral 50 designates any conventional steam boiler heated bythe combustion of a sulfur-containing fuel, e.g. coal, introducedthrough a conduit 52 with air introduced through a pipe 54. Hot flue gascontaining S0 is conducted by a' pipe 56 to an air preheater 58 for heatexchange with the incoming air carried by the pipe 54. The fine gas isthen passed through a pipe 60 to a scrubber 62 for removal of S0 in amanner to be more fully described below. The resulting flue gas ofreduced or zero S0 content is discharged through a stack 64.

The scrubber 62 is any conventional gas-liquid scrubbing tower designedto assure contact of the hot line gas with the selected formate, in thisinstance potassium formate in a molten state, at a temperature betweenthe melting point and 400 F., e.g. 350 F. The hot line gas is scrubbedfree, or substantially so, of S0 in the scrubber 62 by contact with themolten potassium formate. The SO -free gas is discharged through thestack 64 as clean stack gas. Since the stack gas is at an elevatedtemperature, its plume does not fall to ground level, but rises-anddiffuses into the upper atmosphere.

, The chemicaLreaction occurring in the scrubber 62 is thatset forthabove in Equation 1. The CO produced in the reaction is discharged withthe stack gas through stack 64. The reaction is suitably regulated toprovide for the conversion of between about 7 to 25 percent by weight ofthe, formate to the thiosulfate. The solubility of the thiosulfate inthe molten formate is about 7 percent, so that the product leavingthescrubber is in the form of a slurry of the undissolved thiosulfate inthe formate melt. If it is desired to recover any thiosulfate for useper se, for instance asa photographic fixing agent, then the slurry maybe withdrawn from the scrubber 62 by a pipe 66 to a fi1ter68 wherethe-thiosulfate and other entrained solids maybe filtered and dischargedthrough a conduit 70, for further purification of the thiosulfate.

However, we prefer to regenerate formate from the .thiosulfate for reusein the treatment of flue gas. Accordingly, the SO -freethiosulfate-formate slurry is pumped around the, filter 68 by a by-passline 71 to a pipe 72 which leads to the first of two reduction zonessuitably housed in interconnected vessels designated by the numerals 73and 74, respectively, and also identified by the legends Reductor No. 1and-Reductor No. 2, respectively. A suitably regulated stream of C0 andH is fed to each of the Reductors by a main pipe 75 with spur pipelines76 and 77 leading respectively to vessels 73 and 74. The stream of COand H is blended with recycle gas from line 89. The main pipe 75 issupplied with the reducing gas CO and H produced in any suitable manner.The preferred gas composition is'one that has a CO/H mole ratio of about1:1.

Such a gas may be generated in a partial combustion Zone 78 using oxygenfrom line 79 and natural gas from line 80, blended with a CO -richrecycle gas from line 88.

Other suitable means of supplying CO/H may be used, such as partialcombustion of fuel oil, catalytic reforming of natural gas with carbondioxide-steam mixtures and by steam gasification of coal or coal char.

The partial combustor 78 may be operated at the same or preferablysomewhat lower pressure level than the Reductors Nos. 1 and 2. In thelatter case, a compressor, not shown, would be installed in line 75which delivers CO/Hg gas to the regeneration system.

The reactions conducted in the Reductor No. 1 are those set forth in thefollowing equations, where molten potassium formate is the selectedabsorbent:

TABLE IV .-Minimum CO and H Pressure Temperature, E: CO-l-H pressure(p.s.i.)

The reaction conducted in Reductor No. 2 is that set forth below inEquation 9, where the carbonate is converted back to formate.

The preferred operating conditions for this second reduction zone are asfollows: a temperature between 600 and 700 F. and a pressure about 1000p.s.i.g. above the equilibrium pressures given in Table IV. Theregenerated formate, together with unreacted formate, is recycled bypipe 81 to the scrubber 62. The eifluent gases produced in Reductor No.2 are passed to Reductor No. 1 through line 91. The eflluent gases fromReductor No. 1 are passed by pipe 82 to an H S absorber 84 where the H Sis selectively removed from the efiluent gases. The H S is conducted bya pipe 86 to a sulfur recovery plant. The H s-free eflluent gases arerecycled in part by pipes 88 and 89 back to the Reductors Nos. 1 and 2.Another part is passed through pipes 88 and 80 to the partial combustionunit 78 where it is blended with natural gas feed. Finally, some of thegas is purged from the system through line 92 to prevent accumulation ofimpurities.

According to the provisions of the patent statutes, we have explainedthe principle, preferred construction, and mode of operation of ourinvention and have illustrated and described what we now consider torepresent its best embodiment. However, we desire to have it understoodthat, within the scope of the appended claims, the invention may bepracticed otherwise than as specifically illustrated and described.

We claim:

1. A method of reducing the sulfur dioxide content of flue gas to reduceair pollution comprising passing said flue gas containing sulfur dioxideinto contact with a formate selected from the group consisting of sodiumformate, potassium formate, and ammonium formate, in a liquid state, ata temperature above F., but below the decomposition temperature of theselected formate.

2. The method according to claim 1 in which the temperature is below 475F.

3. The method according to claim 2 in which the formate is in aqueoussolution.

4. The method according to claim 3 in which the tem perature is betweenand 225 F.

5. The method according to claim 2 in which the formate is aqueouspotassium formate and the temperature is between and 200 F.

6. The method according to claim 2 in which the formate is in admixturewith the corresponding carbonate.

References Cited UNITED STATES PATENTS 1,036,705 8/1912 Portheim 23-1161,166,160 12/1915 Portheim 23-116 2,010,615 8/1935 Vanderbilt et a1.23-116 2,031,802 2/1936 Tyrer 23-178 2,142,987 1/1939 Bacow et a1 23-1783,411,875 1/1968 Yoshikawa et a1 23-116 3,576,598 4/1971 Plentovich eta1. 23-116 3,584,042 6/ 1971 Yavorsky et a1 23-115 X 3,592,850 7/1971Mazzocco et al. 23-115 X OTHER REFERENCES Goliath et al.: Mechanism ofReduction of Sulfur Dioxide by Formic Acid, Acta Chemica Scandinavica,v01. 16, No. 3, 1962, pp. 570-574.

EARL C. THOMAS, Primary Examiner US. Cl. X.R.

23-1 D, 2 S, 178 S; 423-514; 260-542 52 3 UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent No. 3,687 6l5 Dated Auqllst 29 1972Inventor(s) Everett Gorin and Paul M. Yavorskv It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

In Column 1, line 31: Ser. No. 879,244 should read In Column 2, line 48:After 320F., insert i.e.,

In Column 5, Table II: For "3.3", read -3.38 and (Last column) for "15.7read -l5.72

In Column 7, line 72: "reactions:" should read reaction:

Signed and sealed this 6th day of February 1973.

(SEAL) Attost:

EDWARD MELBTCIERJR. Attesting Officer ROBERT GOTTSCHALK Commissioner ofPatents

